Stirred tank polymerization reactors for producing, for example, ethylene-propylene-diene (EPDM) elastomers are generally known. Such reactors generally comprise a reactor tank, and a stirring assembly including a motor-driven shaft that drives an array of paddles in the interior of the tank. The shaft of the stirring assembly is typically rotatably and sealingly mounted in the reactor tank by means of a mechanical seal. Such seals usually include an outer cylindrical shell that is fixedly mounted in the top wall of the tank, and an inner cylindrical shell that is rotatable within the outer shell and affixed to the motor-driven shaft of the stirring assembly. The inner cylindrical shell generally includes a pair of sealing rings which are spaced apart along its axis of rotation. Each of the sealing rings is typically spring-biased against an annular sealing seat fixedly mounted in the outer sealing ring. A pressurized source of barrier fluid, which may be mineral oil, is connected to an annular space defined between the inner diameter of the outer ring and the outer diameter of the inner ring.
In operation, a flow of monomer is introduced into the interior of the tank along with a catalyst. At the same time, the shaft of the stirring assembly is rotatably driven by a motor so that the paddle array continuously agitates the monomer and catalyst, thereby promoting the reaction of the monomer into polymers such as EPDM elastomers. During this process, the fluid contents of the tank is typically subjected to both elevated pressures, e.g., on the order of 120 bars, which adiabatically generates elevated temperatures, e.g., around 400° F.-500° F. (204° C.-260° C.). In order to prevent the fluid contents of the tank from contaminating the barrier fluid used in the mechanical seal, the barrier fluid is usually pressurized to a level slightly above that of the reaction tank. Moreover, in order to protect the lowermost sealing ring and sealing seat from elevated temperatures generated within the reaction tank, a “seal flush” flow of externally-supplied monomer of about two to three gallons per minute is typically provided at the vicinity of the lowermost sealing ring and sealing seat.
While the aforementioned mechanical seal prevents leakage of the reactor contents during polymer production, the applicants have observed that such seals can effectively function only for a limited period, e.g., about eight or nine months, which often makes the seals the limiting component of reactor overhauls. Consequently, the applicants' experience has been that the reactor would need to be completely shut down every eight or nine months solely for the purpose of replacing the seal. The resulting downtime and disruptions in production significantly increase the costs of the final products.
Close inspection of worn seals by the applicants have revealed the presence of spalling on the interfacing surfaces between the sealing rings and sealing seats. The applicants believe such spalling is caused by the combination of mechanical and thermal stresses that the sealing rings and seats are subjected to during operation. In particular, the applicants have determined that lowermost sealing ring and sealing seat are subjected to high temperatures due to their constant exposure to the fluid contents of the reactor tank (despite the continuous use of the aforementioned seal flush to lower these temperatures), while the uppermost sealing ring and sealing seat are constantly subjected to a high differential pressure (i.e., the difference between the pressure of the barrier fluid and the ambient atmospheric pressure to which the barrier fluid ultimately flows).